Browse By Author ID - Silverberg, Gerald - UNU Collections
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United Nations UniversityenFez 2.1 RC3http://blogs.law.harvard.edu/tech/rssA Note on Michelacci and Zaffaroni, Long Memory, and Time Series of Economic Growth
http://collections.unu.edu/view/UNU:1081
In a recent paper in The Journal of Monetary Economics, Michelacci and Zaffaroni (2000)estimate long memory parameters for GDP per capita of 16 OECD countries. In this note weargue that these estimations are questionable for the purposes of clarifying the time seriesproperties of these data (presence of unit roots, mean reversion, long memory) because theauthors a) filter out a deterministic linear-in-logs trend instead of first-differencing in logs,and manipulate the data in other highly questionable ways, b) rely on the semiparametricGeweke and Porter-Hudak (GPH) method as modified by Robinson, which is known to behighly biased in small samples. We re-examine these results using Beran's nonparametricFGN estimator and Sowell's exact maximum likelihood ARFIMA estimator. These methodsavoid the small-sample bias and arbitrariness of the cut-off parameters of Robinson's methodand allow us to control for short memory effects, although the parametric ARFIMA estimatorintroduces specification problems of its own. We also look at the influence of the choice ofsub-periods on the results. Finally, we apply Robinson's method to our treatment of the dataand show that MZ's results no longer hold, nor are their cut-off parameter and filteringinsensitivity claims substantiated.2013-12-13T13:01:02Z
Silverberg, Gerald
og Verspagen, Bart
A percolation model of eco-innovation diffusion: the relationship between diffusion, learning economies and subsidies
http://collections.unu.edu/view/UNU:372
An obstacle to the widespread adoption of environmentally friendly energy technologies such as stationary and mobile fuel cells is their high upfront costs. While much lower prices seem to be attainable in the future due to learning curve cost reductions that increase rapidly with the scale of diffusion of the technology, there is a chicken and egg problem, even when some consumers may be willing to pay more for green technologies. Drawing on recent percolation models of diffusion by Solomon et al. [7], Frenken et al. [8] and HÃ¶hnisch et al. [9], we develop a network model of new technology diffusion that combines contagion among consumers with heterogeneity of agent characteristics. Agents adopt when the price falls below their random reservation price drawn from a lognormal distribution, but only when one of their neighbors has already adopted. Combining with a learning curve for the price as a function of the cumulative number of adopters, this may lead to delayed adoption for a certain range of initial conditions. Using agent-based simulations we explore when a limited subsidy policy can trigger diffusion that would otherwise not happen. The introduction of a subsidy policy seems to be highly effective for a given high initial price level only for learning economies in a certain range. Outside this range, the diffusion of a new technology either never takes off despite the subsidies, or the subsidies are unnecessary. Perhaps not coincidentally, this range seems to correspond to the values observed for many successful innovations.2013-12-11T16:22:45Z
Cantono, Simona
og Silverberg, Gerald
A percolation model of the product lifecycle
http://collections.unu.edu/view/UNU:34
The product lifecycle model can be understood as a three-stage model of technological development associated with a particular product technology. In the explorative stage many different designs are developed, in the development stage products become standardized into a dominant design, and in the mature stage only incremental changes occur within the dominant design. Although the product lifecycle model is widely accepted and often applied in empirical research, innovation scholars have failed to develop systematic theoretical models that explain the different stages of technological development along the lifecycle. In this study, an attempt is made to contribute to product lifecycle theory by developing a theoretical model based on percolation dynamics. The model combines the concept of increasing returns to adoption with information diffusion among consumers within social networks. The main contribution of the model is that it replicates the three stages of the product lifecycle as an outcome of a single elementary process. The model also replicates the S-shaped diffusion curve and the occurrence of an industry shakeout.2013-12-11T17:24:30Z
Frenken, Koen
og Silverberg, Gerald
og Valente, Marco
Evolutionary Modeling in Economics: Recent History and Immediate Prospects
http://collections.unu.edu/view/UNU:1033
2013-12-13T13:02:43Z
Silverberg, Gerald
Long Memory in Time Series of Economic Growth and Convergence
http://collections.unu.edu/view/UNU:1052
2013-12-13T13:02:03Z
Silverberg, Gerald
og Verspagen, Bart
Long Waves: Conceptual, Empirical and Modelling Issues
http://collections.unu.edu/view/UNU:1131
The theory of long waves is exceptionally fortunate in that, while there is no general consensus that they exist or, assuming that they do, what an appropriate theory should be, due to the unstinting efforts of several researchers, we have encyclopaedic compendia of the literature (Freeman 1996, Reijnders and Lou____ 1999) and a recent valiant attempt to write modern economic history from a long-wave perspective (Freeman and Lou____ 2001). The purpose of this entry is to succinctly review the controversy about what long waves might mean as a phenomenon, how they might be measured and modelled, and where they might fit into an overarching theory of economic dynamics and evolution.2013-12-13T12:41:48Z
Silverberg, Gerald
Self-organization of R&D search in complex technology spaces
http://collections.unu.edu/view/UNU:1188
We extend an earlier model of innovation dynamics based on invasive percolation by adding endogenous R&D search by economically motivated firms. The {0,1} seeding of the technol-ogy lattice is now replaced by draws from a lognormal distribution for technology 'difficulty'. Firms are rewarded for successful innovations by increases in their R&D budget. We compare two regimes. In the first, firms are fixed in a region of technology space. In the second, they can change their location by myopically comparing progress in their local neighborhoods and probabilistically moving to the region with the highest recent progress. We call this the mov-ing or self-organizational regime. We find that as the mean and standard deviation of the log-normal distribution are varied, the relative rates of aggregate innovation switches between the two regimes. The SO regime has higher innovation rates, other things being equal, for lower means or higher standard deviations of the lognormal distribution. This results holds for in-creasing size of the search radius. The clustering of firms in the SO regime grows rapidly and fluctuates in a complex way around a high value which increases with the search radius. We also investigate the size distributions of the innovations generated in each regime. In the fixed one, the distribution is approximately lognormal and certainly not fat tailed. In the SO regime, the distributions are radically different. They are much more highly right skewed and show scaling over at least two decades with a slope of almost exactly one, independently of parame-ter settings. Thus we argue that firm self-organization leads to self-organized criticality.2013-12-13T12:39:11Z
Silverberg, Gerald
og Verspagen, Bart
The size distribution of innovations revisited: an application of extreme value statistics to citation and value measures of patent significance
http://collections.unu.edu/view/UNU:1170
This paper focuses on the analysis of size distributions of innovations, which are known to be highly skewed. We use patent citations as one indicator of innovation significance, constructing two large datasets from the European and US Patent Offices at a high level of aggregation, and the Trajtenberg (1990) dataset on CT scanners at a very low one. We also study self-assessed reports of patented innovation values using two very recent patent valuation datasets from the Netherlands and the UK, as well as a small dataset of patent license revenues of Harvard University. Statistical methods are applied to analyse the properties of the empirical size distributions, where we put special emphasis on testing for the existence of 'heavy tails', i.e., whether or not the probability of very large innovations declines more slowly than exponentially. While overall the distributions appear to resemble a lognormal, we argue that the tails are indeed fat. We invoke some recent results from extreme value statistics and apply the Hill (1975) estimator with data-driven cut-offs to determine the tail index for the right tails of all datasets except the NL and UK patent valuations. On these latter datasets we use a maximum likelihood estimator for grouped data to estimate the Pareto exponent for varying definitions of the right tail. We find significantly and consistently lower tail estimates for the returns data than the citation data (around 0.7 vs. 3-5). The EPO and US patent citation tail indices are roughly constant over time (although the US one does grow somewhat in the last periods) but the latter estimates are significantly lower than the former. The heaviness of the tails, particularly as measured by financial indices, we argue, has significant implications for technology policy and growth theory, since the second and possibly even the first moments of these distributions may not exist.2013-12-13T12:39:57Z
Silverberg, Gerald
og Verspagen, Bart
When is a Wave a Wave? Long Waves as Empirical and Theoretical Constructs from a Complex Systems Perspective
http://collections.unu.edu/view/UNU:1187
While long waves have been seriously discussed by economists for almost one hundred years, to date there is no scientific consensus that particular frequency components are in any way privileged in the undoubtedly fluctuating history of modern economic and political development. This is disappointing for two reasons. First, the demonstration that robust, well-defined periodic components existed would present us with a plausible tool for forecasting. And second, they could (and their purported existence has variously been thought to) provide insight into underlying causal mechanisms that generate the observed patterns. The data, I argue, only provide support for a continuous spectral pattern of a power law, 1/fa. This is borne out in the paper by the analysis of political indicators such as the newly revised Modelski/Thompson sea power index and the Levy great powers conflict data. Claims for underlying low-dimensional chaos are only partly substantiated. Individual peaks at various frequencies in the spectrum are probably only due to 'random noise' factors unique to segments of the record and not robust across countries and historical episodes. While one could then play the game of finding ad hoc explanations for why the 'K-wave' did not take its expected form in this or that century, from the perspective of the theory of complex dynamics it seems more plausible to conclude that a periodic model is not appropriate. Rather, the underlying model is more likely to be of the self-organized criticality or percolation type, characterized by power-law or fractal behavior rather than well-defined periodicity. I highlight some features common to several models of innovation/ economic dynamics and war/hegemonic cycles, such as highly clustered but nonperiodic critical events and resulting long life cycles of rise and decline, that may serve as a plausible explanatory mechanism for this 'revisionist' interpretation of the empirical record on long waves.2013-12-13T12:39:13Z
Silverberg, Gerald